1. Field of the Invention
The present invention relates to an anti-reflection film material which is suitably used for fine processing in production processes of a semiconductor device or the like, particularly an anti-reflection film material of which a main component is a polymer compound which contains silicon atoms. Furthermore, the present invention relates to a substrate having an anti-reflection film suitable for exposure with a far ultraviolet ray, ArF excimer laser light (193 nm), F2 laser light (157 nm), Kr2 laser light (146 nm), Ar2 laser light (126 nm), or the like, and a method for forming a pattern on the substrate.
2. Description of the Related Art
With a tendency of high integration and high-speed of LSI, a finer pattern rule is needed in recent years, and in lithography using optical exposure which is used as a general technique at present, an essential resolution derived from a wavelength of a light source has almost reach the limit.
The optical exposure using g line (436 nm) or i line (365 nm) of a mercury-vapor lamp as a light source for lithography when a resist pattern is formed, has been used widely. It has been considered that a method of using an exposure light with a shorter wavelength is effective as a means for a further finer pattern. For this reason, for example, the KrF excimer laser (248 nm) with a short wavelength has come to be used as an exposure light source instead of i line (365 nm), for mass-production process of the 64 M bit DRAM processing method. However, a light source with far shorter wavelength is needed for manufacture of DRAM with a degree-of-integration of 1 G or more which needs a still finer processing technique (for example, a processing dimension is 0.13 μm or less), and lithography using the ArF excimer laser (193 nm) has been especially examined.
In the early stage of lithography using the KrF excimer laser (hereafter referred to as KrF lithography), there has been developed a stepper (aligner) in which an achromatic lens or a reflecting optical system and a broadband light are combined. However, the combination of the narrow spectrum laser light and the refracting-optical-system lens has been dominant, since an accuracy of the achromatic lens or an aspherical surface reflecting optical system is not enough. Generally, it is a phenomenon which has been known well for many years that the incident light and reflected light from a substrate interfere with each other in exposure with a single wavelength, which may lead to generation of a standing wave. Moreover, it is also known that the phenomenon called halation that light is condensed or scattered due to irregularity of a substrate may be caused. Both of the standing waves and the halation may cause variation of a dimension such as line width of a pattern or the like, change in a shape, or the like. Use of a coherent monochromatic light amplifies the standing wave and the halation further as wavelength gets shorter. For this reason, there have been proposed a method of adding a light absorber to a photoresist film material and a method of covering an upper surface of a photoresist film and a surface of a substrate with an anti-reflection film, as a method of suppressing the halation and the standing wave.
However, there was caused a problem that a shape of the resist pattern turns into a taper in the method of adding a light absorber. As a wavelength gets shorter and a pattern gets finer in recent years, the problem of variation of pattern dimension due to the standing wave and the halation has become serious, and it has become impossible to fully solve the problem by the method of adding a light absorber.
In the method of covering the upper surface of the photoresist film with the anti-reflection film, the anti-reflection film (hereafter referred to as the upper layer transmission type anti-reflection film) is effective only in reduction of a standing wave in principle, and it is not effective in the halation. Moreover, it is ideal that the refractive index of the upper layer transmission type anti-reflection film for erasing a standing wave completely is identical with a square root of the refractive index of the photoresist film. Accordingly, when the refractive index is 1.8 which is that of a photoresist film made of poly hydroxy styrenes used in KrF lithography, 1.34 is an ideal value.
When the refractive index is 1.6 which is that of an alicyclic acrylic photoresist film used for the lithography using a ArF excimer laser (hereafter referred to as ArF lithography), the ideal value is 1.27. A perfluoro material is the only material which has such a low refractive index. However, it is necessary that the anti-reflection film is a water-soluble material, since it is more advantageous for the processes that the anti-reflection film can be exfoliated at the time of alkali development. If a hydrophilic substituent is introduced in order to make a highly hydrophobic perfluoro material water-soluble, a refractive index will be increased, and the value in KrF lithography gets around 1.42, and the value in ArF lithography gets around 1.5. Accordingly, when a pattern is formed with a processing dimension of 0.20 μm or less in KrF lithography, it is impossible to suppress the influence of a standing wave only by the combination of a light absorber and the upper layer transmission type anti-reflection film. In ArF lithography, the effectiveness of the upper layer transmission type anti-reflection film can hardly be expected for the reason for the above, and also in KrF lithography, control of a line width becomes severe because of further decrease of the line width in future.
Then, there has become necessary a method of covering a surface of a substrate with an anti-reflection film, i.e., a method of forming an anti-reflection film as a ground of a photoresist film.
In the case that the layer under an anti-reflection film as a ground of the photoresist film is a high reflective substrate, such as poly silicon, aluminum, or the like, the anti-reflection film can reduce reflection from the substrate to 1% or less by being formed of the material having an optimal refractive index (n value) and an optimal extinction coefficient (k value) with a suitable thickness, and thereby quite high effect can be achieved.
FIG. 1 is a graph which shows a relation between a thickness of the anti-reflection film and a reflectivity at a wavelength of 193 nm. FIG. 1 shows that, for example, in the case that the refractive index of the photoresist film is 1.8 at a wavelength of 193 nm, if the refractive index of the anti-reflection film under the photoresist film (a real part of a refractive index) n is 1.5, the extinction coefficient(a imaginary part of a refractive index) k is 0.5, and a thickness is 42 nm, a reflectivity will become 0.5% or less.
However, in the case that there is a level difference in the ground substrate, a thickness of the anti-reflection film is sharply changed at the level difference. Although the reflection preventive effect in the first base with a thickness of 40–45 nm where the interference effect is strong is high, a reflectivity is sharply changed due to variation of a thickness, since the reflection preventive effect of the anti-reflection film uses not only an absorption of light but the interference effect, as shown in FIG. 1.
Then, there has been proposed the material in which a thickness variation on a level difference is suppressed by increasing a molecular weight of a base resin used for the anti-reflection film material, and thereby a conformability is improved (for example, see Japanese Patent Application Laid-open (KOKAI) No. 10-69072). However, if the molecular weight of the base resin becomes high in this case, there may be caused a problem that a pinhole is easily generated after spin coating, a problem that it becomes impossible to be filtered, a problem that viscosity change will be caused with time and a thickness will be varied, and a problem that a crystal is deposited at a tip of a nozzle. Moreover, conformability can be achieved only where a level difference is comparatively low.
Then, there can be considered a method of adopting a thickness more than that of the 3rd base (170 nm or more) where variation of a reflectivity due to variation of a thickness is comparatively small from FIG. 1. In this case, if k value is between 0.2–0.3, and a thickness is 170 nm or more, the variation of a reflectivity due to variation of a thickness will be small, and a reflectivity can be suppressed to 2.0% or less.
Moreover, in the case that the ground of the anti-reflection film is a transparent film, such as an oxide film, a nitride film or the like, and there is a level difference under the transparent film, even though the surface of the transparent film was made flat by CMP (Chemical Mechanical Polishing) or the like, the thickness of the transparent film is varied. In this case, although it is possible to make the thickness of the anti-reflection film on the transparent film constant, the thickness of the film with which the reflectivity is minimum in FIG. 1 will shift by the thickness of the transparent film with a period of λ/2n (λ: exposure wavelength and n: refractive index of the transparent film at the exposure wavelength), when the thickness of the transparent film under the anti-reflection film is varied. If the thickness of the anti-reflection film is 55 nm which provides the minimum reflectivity in the case that the ground is a reflective film, a portion where a reflectivity is high is generated with change of a thickness of the transparent film. In this case, it is necessary to make a thickness of an anti-reflection film thick as 170 nm or more as described above, in order to stabilize the reflectivity to variation of a thickness of a transparent film as a ground.
The material of the anti-reflection film as described above can be roughly classified into an inorganic material and an organic material.
Example of the inorganic material may be a SiON film. Since it is formed by CVD (Chemical Vapor Deposition) with a mixed gas of silane and ammonia or the like and the etch selectivity to the photoresist film is large, it has an advantage that etching load to the photoresist film is small. However, since it is hardly exfoliated, application thereof is limited. Moreover, since it contains a nitrogen atom and is basic, there is also a disadvantage that the footing profile is likely caused in the case of a positive resist, and an undercut profile is easily caused in the case of a negative resist.
The organic material is advantageous, since it can be formed by a spin coating, it does not need special equipments, such as CVD system, sputtering system or the like, it can be stripped together with the photoresist film, footing profile or the like is not generated, the shape is simple, and it has an excellent adhesion property with the photoresist film. Accordingly, a lot of anti-reflection film materials based on organic materials were proposed. For example, there were proposed a condensation product of a diphenylamine derivative and a formaldehyde modified melamine resin, those consisting of an alkali soluble resin and a light absorber (for example, see Japanese Patent publication No.7-69611), those containing a reaction product of a maleic anhydride copolymer and diamine type light absorber (for example, see U.S. Pat. No. 5,294,680 specification), those containing a resin binder and a methylol melamic heat crosslinking agent (for example, see Japanese Patent Application Laid-open (KOKAI) No. 6-118631), the acrylate resin base type which has a carboxylic acid group, an epoxy group, and a light-absorption group in the same molecule (for example, see Japanese Patent Application Laid-open (KOKAI) No. 6-118656), those consisting of methylol melamine and a benzophenone light absorber (for example, see Japanese Patent Application Laid-open (KOKAI) No. 8-87115), those in which a low molecule light absorber is added to a polyvinyl alcohol resin (for example, see Japanese Patent Application Laid-open (KOKAI) No. 8-179509). All of the anti-reflection film material using these organic materials as a base is produced by a method of adding a light absorber into a binder polymer, or introducing a light-absorption group into a polymer as a substituent. However, since many of the light absorbers have an aromatic group or a double bond, dry etch resistance is raised by addition of the light absorber, and there is a disadvantages that a dry etch selectivity to the photoresist film is not so high. Since a pattern tends to be finer, a photoresist film tends to be thin, and furthermore, an acrylic or an alicyclic polymer will be used as a photoresist film material in ArF lithography of the next generation, etching resistance of the photoresist film tends to be lowered. Furthermore, there is also a problem that a thickness of the anti-reflection film needs to be increased, as explained above. For these reasons, etching is becoming a serious problem, and thus the anti-reflection film with high etch selectivity to the photoresist film, namely the anti-reflection film of which an etch rate is high when the anti-reflection film is etched using the photoresist film as a mask, has been desired.
Furthermore, the light absorber for affording the optimal absorbancy index in an anti-reflection film has been examined. Especially, there has been proposed an anthracene type in KrF lithography, and a phenyl type in ArF lithography. However, as explained above, they are also the substituents which have an excellent dry etching resistance. Accordingly, even if a polymer having low etching resistance such as an acrylic resin is used as a polymer backbone from which a die is suspended, there is a practical limit.
Furthermore, the photoresist film tends to be thinner with progress of tendency of high resolution in recent years. Although the improvement in etching resistance of the photoresist film is needed as a film gets thin, it is not enough at present. Then, a hard mask method is used as a pattern transfer method of a thin photoresist film.
As a hard mask, a SiO2 film has been examined when a substrate to be processed is poly silicon (p-Si), and SiN, W—Si, amorphous Si, or the like have been examined when a substrate to be processed has a SiO2 film. Furthermore, a hard mask made of a SiON film which also has a function as an anti-reflection film has been proposed (for example, see SPIE2000 Vol.4226 p93). In a hard mask method, there are a case where a hard mask remains and a case where a hard mask is stripped. In the case that a ground is an insulator film such as a SiO2 film, it needs to be stripped, since especially, W—Si and an amorphous Si film are good conductive film. When a hard mask is a SiN film, it is not necessary to be stripped in some cases, since it is an insulator layer. However, since it has a similar composition of elements to SiO2, there is a disadvantage that the etch selectivity which is an original function as a hard mask is low.